Effect of Dielectric Constant on the Interaction Between Charged Macroions in Asymmetric Electrolyte

The mean force between two highly like-charged macroions in the presence of monovalent counterions and added multivalent salt, within solvents of varying dielectric constants has been studied using Monte Carlo simulations. Without additional salt, the mean force is strongly repulsive at all macroion separations in solvents with a dielectric constant ϵr ≥ 30. However, in solvents having ϵr ≤ 30, the macroions experience effective attraction, indicating that attractive interactions between highly charged macroions can occur even without multivalent salt in nonpolar solvents with low dielectric constants. At a multivalent counterion-to-macroion charge ratio of β = 0.075, the mean force becomes attractive at short separations in solvents with ϵr = 54 containing 1:3 salt, as well as in all solvents with 1:5 salt, while still exhibiting significant repulsion at longer separations. In contrast, for solvents with 1:3 salt and dielectric constants ϵr = 68 and ϵr = 78.4, the mean force turns attractive at a higher salt concentration, around β = 0.225. The shift of the mean force to an attractive state at short separations signifies charge inversion on the macroion surface when a sufficient amount of salt is present. At a stoichiometric ratio of multivalent counterions, long-range repulsion vanishes, and attraction becomes significant. However, with excess salt, the strength of the attractive mean force diminishes. Additionally, the attractive force at a given salt concentration increases as the dielectric constant decreases and is stronger in systems with 1:5 salt than in those with 1:3 salt.